1. Field of the Invention
The present invention relates to a method and a single photon emission computerized tomographic (SPECT) apparatus for reconstructing a SPECT image by receiving radiation emitted from a radio isotope injected into a biological body under medical examination. More specifically, the present invention is directed to a method and a SPECT apparatus capable of removing a ring-shaped artifact and of correcting sensitivity with employment of a fan-beam collimator and a gamma camera.
2. Description of the Related Art
In general, a fan-beam collimator employed a single photon emission computerized tomographic apparatus (referred to as a "SPECT" apparatus hereinafter) has both spatial resolution and a sensitivity characteristic as represented in FIG. 1 and defined as follows. That is, the spatial resolution and sensitivity characteristic of such a fan-beam collimator are determined based upon a function between a distance "b" measured from a surface of this collimator and an angle ".THETA." with respect to a focal point of the fan beam. As a result, a sensitivity profile curve of the fan-beam collimator is represented in FIG. 2 in which a counted value becomes high at a center portion of the reconstructed SPECT image, and the counted values become rapidly low depending upon distances apart from the center portion along an X-direction. Also, if the fan-beam collimator owns low focusing precision, uneven or unbalanced sensitivity occurs. Further if machining precision of the collimator is lowered, a ring-shaped artifact may happen to occur in a SPECT image. Both of the uneven sensitivity and ring-shaped artifact are superimposed on the sensitivity profile curve.
Under such circumstances, in the conventional SPECT apparatus, as the reconstruction algorithm, one method for correcting only the spatial resolution has been proposed as an image reconstruction algorithm based upon a weight back-projection method for back-projecting a weight coefficient as represent in FIG. 3. Furthermore, another conventional SPECT system has been proposed in, for instance, IEEE Transaction on Nuclear Science, "TRIANGULAR SPECT SYSTEM FOR 3-D TOTAL ORGAN VOLUME IMAGING DESIGN CONCEPT AND PRELIMINARY IMAGING RESULTS" written by C. B. Lim et al., vol NS-32, No. 1, February 1985. In this SPECT system, the radio isotope is stored with a phantom having a diameter equal to an effective field and then a SPECT image is once acquired. Based upon the SPECT image containing the ring-shaped artifact component, the uneven sensitivity, and the uneven SPECT value caused by the scattering and absorption of the gamma ray, a memory table is produced by the reverse SPECT values as correction data, and the subsequent SPECT image data are corrected based on these correction data read out from the memory table.
However, there are the following problems in these conventional correcting methods for the SPECT imaging apparatuses. That is, while the sensitivity component caused by the fan-shaped collimator is not separated from the component of the ring-shaped artifact, both the sensitivity correcting and ring-shaped artifact removing operations are simultaneously carried out for the reconstructed SPECT image data. As a consequence, both the sensitivity correction for the SPECT system (especially fan-beam collimator) and ring-shaped artifact elimination may not be precisely executed because the resultant SPECT image data still contains uneveness caused by the scattering phenomenon of the gamma ray.